Fusion of macrophages is a critical step in the formation of multinucleate osteoclasts, which are responsible for bone loss associated with osteoporosis, rheumatoid arthritis and periodontal disease. Macrophages also fuse in chronic inflammatory reactions and in tumors where they form multinucleate giant cells. Fusion is a regulated developmental event that allows macrophages to become multinucleate and acquire a new function. Exosomes are small virus-like particles released by cells, including macrophages, whose structure has been well characterized in tumors and fluids, but whose functions remain poorly understood. Since all cells release exosomes, we hypothesize that exosomes released by fusing macrophages may participate in the fusion process. In addition, since exosomes are present in body fluids and are used as biomarkers to detect diseases such as cancer, we also hypothesize that exosomes released by fusing human osteoclasts express specific proteins and RNAs that can be used as biomarkers to assess osteoclast formation leading to bone loss. We therefore propose to characterize morphologically and biochemically the exosomes released by fusing human monocytes using microscopy, proteomics, RNA microarray analysis and microRNA profiling. PUBLIC HEALTH RELEVANCE: Osteoclasts are multinucleate cells that form by fusion of macrophages and are responsible for bone loss associated with osteoporosis and rheumatoid arthritis. Exosomes are virus-like membrane bound vesicles released by cells, including macrophages, which are present in extracellular fluids, including blood and milk. We propose to isolate and characterize morphologically and biochemically exosomes released by fusing human monocytes, which form osteoclasts, to improve our understanding of osteoclastogenesis and to potentially use them as biomarkers to assess osteoclast-mediated bone loss.